The enzyme hypoxanthine phosphoribosyltransferase (HPRT) is constitutively expressed at low levels in all tissues except in parts of the central nervous system where it is expressed at five- to eight-fold higher levels. Total deficiency of this enzyme in humans causes Lesch-Nyhan syndrome, an X-linked neurological disorder characterized by mental retardation, chorioathetosis, self-mutilative behavior and hyperuricemia. This application proposes to identify and precisely delineate the cis-acting positive and negative regulatory sequences of the HPRT gene. Deletion analysis and in vitro mutagenesis will be used to study the functional role of these sequences in HPRT expression. Trans- acting factors interacting with these sequences will be identified and their interaction with the sequence characterized. These factors will be purified and cDNA clones expressing them will be isolated in order to understand their role in the regulation of the HPRT gene and other functionally-related genes. Transgenic mice that harbor a reporter gene, the E. Coli beta-galactosidase gene linked to the various cis-acting regulatory sequences of the HPRT gene, will be created and expression of the reporter gene in various tissues, including the brain, examined to determine the in vivo role of the regulatory sequences. These studies will facilitate our understanding of tissue-differential gene expression, housekeeping gene expression and potentially define the factors causing increased central nervous system expression of the HPRT gene. This latter information will be of value in the context of knowing the central nervous system dysfunction in Lesch-Nyhan syndrome and in gene therapy for this disease.